Lens system for optical beam scanner

ABSTRACT

The present invention is directed to a lens system for an optical beam scanner which scans a plurality of optical beams. The lens system comprises: a first lens group having an entrance pupil, which includes first and third lenses in the form of meniscus having a positive power, respectively, a fourth lens having a positive power, and a second lens having a negative power; and a second lens group including a fifth lens having a positive power. The first to fifth lenses are successively disposed in order from the entrance pupil side. Accordingly, the lens system has a telecentric and an f-sinθ characteristic.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a lens system, and more particularly, to a lens system which is applied to an optical beam scanner, such as a laser printer, which scans a plurality of optical beams on a recording surface.

2. Description of the Prior Art

Japanese Patent Laying-Open Gazettes Nos. 195211/1984 and 267910/1988 disclose lens systems which can be applied to an optical beam scanner such as a laser printer, respectively.

FIG. 1 illustrates the structure of a conventional telecentric f-θ lens system, which is disclosed in the former gazette Japanese Patent Laying-Open Gazette No. 195211/1984). As shown in FIG. 1, the conventional telecentric f-θ lens system is formed with a negative meniscus lens L₁₂ whose concave surface S₁₂ is directed toward an entrance pupil EP, positive meniscus lenses L₁₃ and L₁₄ and a positive plano-convex lens L₁₅, and these lenses L₁₂ to L₁₅ are arranged in order from the entrance pupil EP side toward a recording surface RS side.

The aforementioned lens system satisfies:

    h=f·θ

assuming that h represents a distance from an optical axis Z of the lens system to a beam spot on the recording surface RS, f represents the focal length of the lens system and θ represents an incident angle of the beam upon the lens system. A laser beam is generally deflected by a polygon mirror, so that the laser beam is impinged on the recording surface RS through the lens system. Since the polygon mirror is rotated at a constant angular velocity, the beam spot moves on the recording surface RS at a constant speed.

Further, the lens system has a telecentric characteristic that all principal rays substantially vertically enter the recording surface RS, so that an image size in a main scanning direction does not vary even if the recording surface RS is displaced in the direction of the optical axis Z, for example, as is well known in the art.

Therefore, a telecentric f-θ lens system is employed in a laser printer etc. particularly requiring that a laser beam is projected onto the recording surface in high accuracy.

FIG. 2 shows a schematic illustration of the optical path deflected by a deflector 1. A plurality of laser beams B₀ to B_(n) are projected onto a point O of the deflector 1 which rotates in the forward and reverse directions around a Y-axis, while laser beams b₀ to b_(n) deflected by the deflector 1 are directed toward a recording surface RS (X-Y plane) through a lens system 2a. In the figure, symbols i₀ to i_(n) represent beam spots of the laser beams b₀ to b.sub. on the recording surface RS, respectively.

In the case that an f-θ lens system shown in FIG. 1 is employed as the lens system 2a, the following equation is obtained: ##EQU1##

Where y is a distance between the beam spots i₀ and i_(n), f is a focal length of the f-θ lens system 2a, α is an angle between the laser beam B_(n) and an optical axis X, and β is an angle between the laser beam b₀ and an optical axis Z of the f-θ lens system 2a.

When the deflector 1 rotates around the Y-axis, the angle β varies corresponding to the rotation of the deflector 1, whereby the distance y varies with the angle β. Therefore, the locus of the laser beams focusing on the recording surface RS by the f-θ lens system 2a will not straight as shown in FIG. 3, so that the quality of an image recording on the recording surface RS is lowered.

FIG. 4 illustrates the structure of an f-sinθ lens system, which is disclosed in latter gazette (Japanese Patent Laying-Open Gazette No. 267910/1988). As shown in FIG. 4, the f-sinθ lens system is formed with a first lens group G₁ having a negative focal length and a second lens group G₂ having a positive focal length, to satisfy:

    h=f·sinθ

where h is a distance between an optical axis Z of the f-sinθ lens system and a beam spot b on a recording surface RS, f is a focal length of the f-sinθ lens system and θ is an incident angle that the optical beam entering the lens system forms with respect to the optical axis Z thereof.

In the case that the f-sinθ lens system 2 is used instead of the lens system 2a as shown in FIG. 2, the following equation is obtained:

    y=f·sinα

As understood the above equation, the distance y is a constant even if the deflector 1 rotates in the forward and reverse direction around the Y-axis. Therefore, the loci of the plural laser beams will be straight as shown in FIG. 5, when the laser beams are directed to the recording surface RS through the f-sinθ lens system.

However, the f-sinθ lens system shown in FIG. 5 has no telecentric characteristic, so that an image size in a main scanning direction varies with the displacement of the recording surface RS in the direction of the optical axis Z of the f-sinθ lens system. Therefore, the lens system can't be employed in the optical beam scanner such as a laser printer requiring that the image can be recorded on the recording surface RS in high accuracy.

SUMMARY OF THE INVENTION

The present invention is directed to a lens system for an optical beam scanner which scans a plurality of optical beams on a recording surface. The lens system comprises: (a) a first lens group having an entrance pupil, which includes (a-1) a first lens in the form of meniscus having a positive power and having a concave surface which is directed to the entrance pupil, (a-2) a second lens having a negative power, (a-3) a third lens in the form of meniscus having a positive power, and a fourth lens having positive power; and (b) a second lens group including a fifth lens having a positive power. Further, the first through fifth lenses are successively disposed in order from the entrance pupil side.

In an aspect of the present invention, the above lens system satisfies following:

    0.6<(r.sub.2 /r.sub.3)≦2.1

    -0.43<(f.sub.2 /f)<-0.21

    1.3<(f.sub.5 /f)≦2.6

where r₂ is a radius of curvature of a surface of the first lens, which is directed to the recording surface; r₃ is a radius of curvature of a surface of the second lens, which is directed to the entrance pupil f₂ is a focal length of the second lens; f₅ is a focal length of the fifth lens; and f is a focal length of the lens system.

Further, the present invention is directed to an optical beam scanner for scanning a plurality of optical beams on a recording surface. The optical beam scanner comprises: a light source for generating the plurality of optical beams; a deflector for deflecting the optical beams outgoing from the light source; and a lens system for imaging the optical beams deflected by the deflector on the recording surface. The lens system comprises: (a) a first lens group having an entrance pupil, which includes (a-1) a first lens in the form of meniscus having a positive power and having a concave surface which is directed to the entrance pupil, (a-2) a second lens having a negative power, (a-3) a third lens in the form of meniscus having a positive power, and a fourth lens having positive power; and (b) a second lens group including a fifth lens having a positive power. Further, the first through fifth lenses are successively disposed in order from the entrance pupil side.

A principal object of the present invention is to provide a lens system which has a telecentric property and an excellent f-sinθ characteristic.

Another object of the present invention is to provide an optical beam scanner which can scan the plurality of optical beams on the recording surface in high accuracy while improving a quality of an image formed on the recording surface.

These and other objects, features, aspects and advantages of the present invention will become more apparent from the following detailed description of the present invention when taken in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates the structure of a conventional telecentric f-θ lens system;

FIG. 2 is a schematic illustration of optical paths deflected by a deflector;

FIG. 3 is a diagram showing a scanning locus made by the telecentric f-θ lens system where a plurality of optical beams are applied;

FIG. 4 illustrates the structure of a conventional f-sinθ lens system;

FIG. 5 is a diagram showing a scanning locus made by the f-sinθ lens system where a plurality of optical beams are applied;

FIG. 6 illustrates a preferred embodiment of a lens system according to the present invention;

FIGS. 7A, 7B and 7C illustrate spherical aberration. astigmatism and f-sinθ characteristic of a first example, respectively;

FIGS. 8A, 8B and 8C illustrate spherical aberration, astigmatism and f-sinθ characteristic of a second example, respectively;

FIGS. 9A, 9B and 9C illustrate spherical aberration, astigmatism and f-sinθ characteristic of a third example, respectively;

FIGS. 10A, 10B and 10C illustrate spherical aberration, astigmatism and f-sinθ characteristic of a fourth example, respectively;

FIGS. 11A, 11B and 11C illustrate spherical aberration, astigmatism and f-sinθ characteristic of a fifth example, respectively;

FIGS. 12A, 12B and 12C illustrate spherical aberration, astigmatism and f-sinθ characteristic of a sixth example, respectively;

FIGS. 13A, 13B and 13C illustrate spherical aberration, astigmatism and f-sinθ characteristic of a seventh example, respectively and

FIG. 14 is a perspective view showing an optical system of an optical beam scanner to which the lens system structured along FIG. 6 is applied.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 6 illustrates a preferred embodiment of a lens system according to the present invention. The lens system is formed with a rear lens group R_(L) and a front lens group F_(L), which are arranged in order from a recording surface RS side at a prescribed interval.

The front lens group F_(L) is formed with first, third and fourth meniscus lenses L₁, L₃ and L₄ having positive power and a second lens L₂ having negative power, as shown in FIG. 6. The first to fourth lenses L₁ to L₄ are arranged in order from an entrance pupil EP side at prescribed intervals, while all of concave surfaces S₁, S₃, S₅ and S₇ of the first to fourth lenses L₁ to L₄ are directed toward the entrance pupil EP.

On the other hand, the rear lens group R_(L) is formed with a fifth lens L₅ having positive power, and a convex surface S₁₀ of the fifth lens L₅ is directed toward the recording surface RS.

The lens system having the aforementioned structure satisfies:

    0.6<(r.sub.2 /r.sub.3)≦2.1                          (1)

    -0.43<(f.sub.2 /f)<-0.21                                   (2)

    1.3<(f.sub.5 /f)≦2.6                                (3)

where r₂ is a radius of curvature of a surface S₂ of the first lens L₁, which is directed toward the recording surface RS; r₃ is a radius of curvature of the surface S₃ of the second lens L₂ ; f₂ is a focal length of the second lens L₂ ; f₅ is a focal length of the fifth lens L₅ ; and f is a focal length of the lens system.

The inequality (1) shows the condition for correcting the spherical aberration, astigmatism and curvature of image surface. That is, when the value (r₂ /r₃) is not more that 0.6, both of the meridional and sagittal image surfaces are inclined toward the negative (-) side while correction of the spherical aberration is insufficient. When value (r₂ /r₃) is more than 2.1, the spherical aberration is excessively corrected while the meridional image surface is inclined toward the positive (+) side, whereby astigmatism at an end of a field angle is increased. Further, the f-sinθ characteristic is degraded also.

The inequality (2) shows the condition for the curvature of image surface. That is, when the value (f₂ /f) is not more than -0.43, the curvature of the sagittal image surface is increased. When the value (f₂ /f) is not less than -0.21, both of the meridional and sagittal image surfaces are inclined toward the negative (-) side.

The inequality (3) shows the condition for the curvature of image surface and the condition that the lens system has a telecentric property. That is, when the value (f₅ /f) is not more that 1.3, the meridional image surface is inclined toward the negative (-) side while the telecentric characteristic is degraded. On the other hand, when the value (f₅ /f) is more than 2.6, the sagittal image surface is inclined in spite of improving the telecentric characteristic of the lens system.

When, to the contrary, a lens system is formed with the first to fifth lenses L₁ to L₅ and the lens system satisfies the inequality (1) to (3) (Examples 1 to 8 described below), the lens system which has both of telecentric and f-sinθ characteristics can be obtained, which further has the following characteristics;

(1) F-number is not more than 20;

(2) spherical aberration, astigmatism etc. are small; and

(3) image surface is flat.

Example 1

Table 1 shows lens data of a first example according to a lens system structured along FIG. 6.

                  TABLE 1                                                          ______________________________________                                         i      r.sub.i        d.sub.i  n.sub.i                                         ______________________________________                                         1      -23.26         1.72     1.77861                                         2      -20.56         7.61                                                     3      -17.10         5.00     1.61655                                         4      -186.05        1.90                                                     5      -105.55        7.92     1.77861                                         6      -35.28         0.12                                                     7      -106.55        6.00     1.77861                                         8      -41.31         122.10                                                   9      ∞        13.59    1.51462                                         10     -98.17                                                                  ______________________________________                                    

Referring to Table 1, symbol r_(i) (i=1, 2, . . . , 10) represents radius of curvature of lens surfaces S_(i), such that r₁ represents the radius of curvature of the concave surface S₁ of the first lens L₁ and r₂ represents the radius of curvature of the convex surface S₂ thereof directed toward the recording surface, for example. Symbol d_(i) (i=1, 3, 5, 7, 9) represents thickness values of the first to fifth lenses L₁ to L₅, respectively. On the other hand, d_(i) (i=2, 4, 6, 8) represents spaces between pairs of the first to fifth lenses L₁ to L₅, respectively, such that d₂ represents the space between the first and second lenses L₁ and L₂, for example. Symbol n_(i) (i=1, 3, 5, 7, 9) represents indexes of refraction of the first to fifth lenses L₁ to L₅ with respect to a line d (587.6 nm).

The focal length f, the F-number F_(NO) etc. of the lens system having the aforementioned lens data are set as shown in Table 2.

                  TABLE 2                                                          ______________________________________                                         f        F.sub.NO      λ d.sub.0                                        ______________________________________                                         100.00   20.00         632.8 nm 10.00                                          ______________________________________                                    

Referring to Table 2, λ represents a wavelength of a laser beam, and d₀ represents a space between the entrance pupil EP and the concave surface S₁ of the first lens L₁. The focal length f₂, f₅ of the second and fifth lens L₂, L₅ is -30.9, 190.8, respectively, although this is not shown in Tables 1 and 2.

Thus,

    r.sub.2 /r.sub.3 =1.202

    f.sub.2 /f=-0.309

    f.sub.5 /f=1.908

and it is apparent that the lens system satisfies the inequalities (1) to (3).

FIGS. 7A, 7B and 7C illustrate spherical aberration, astigmatism and f-sinθ characteristic of the lens system having the aforementioned structure respectively. The results shown in these figures and FIGS. 8A-13A, 8B-13B and 8C-13C, which are hereinafter described in detail, have been obtained with respect to light of 632.8 nm in wavelength.

In each of FIG. 7B and FIGS. 8B, 9B, 10B, 11B, 12B and 13B, symbol S denotes a sagittal image surface and symbol M denotes a meridional image surface, respectively. The f-sinθ characteristic shown in each of FIGS. 7C, 8C, 9C, 10C, 11C, 12C and 13C is obtained by the following equation in which θ represents an angle of incidence of the optical beam upon the lens system and h represents a distance between a beam spot on a recording surface and an optical axis of the lens system: ##EQU2##

Example 2

Table 3 shows lens data of a second example according to the lens system.

                  TABLE 3                                                          ______________________________________                                         i      r.sub.i        d.sub.i  n.sub.i                                         ______________________________________                                         1      -10.50         1.05     1.77861                                         2      -10.85         14.59                                                    3      -17.90         1.00     1.61655                                         4      -201.87        1.27                                                     5      -60.73         3.31     1.77861                                         6      -30.68         0.10                                                     7      -456.16        5.50     1.77861                                         8      -35.92         106.99                                                   9      ∞        12.68    1.51462                                         10     -104.67                                                                 ______________________________________                                    

The focal length f, the F-number F_(NO) etc. of the lens system having the aforementioned lens data are set as shown in Table 4.

                  TABLE 4                                                          ______________________________________                                         f        F.sub.NO      λ d.sub.0                                        ______________________________________                                         100.00   20.00         632.8 nm 11.00                                          ______________________________________                                    

The focal length f₂, f₅ of the second and fifth lens L₂, L₅ is -31.9, 203.4, respectively, although this is not shown in Tables 3 and 4.

Thus,

    r.sub.2 /r.sub.3 =0.606

    f.sub.2 /f=-0.319

    f.sub.5 /f=2.034

and it is apparent that the lens system satisfies the inequalities (1) to (3).

FIGS. 8A, 8B and 8C illustrate spherical aberration, astigmatism and f-sinθ characteristic of the lens system having the aforementioned structure, respectively.

Example 3

Table 5 shows lens data of a third example according to the lens system.

                  TABLE 5                                                          ______________________________________                                         i      r.sub.1        d.sub.i  n.sub.i                                         ______________________________________                                         1      -48.13         3.24     1.77861                                         2      -40.23         10.45                                                    3      -19.16         1.00     1.61655                                         4      -367.82        1.74                                                     5      -42.17         6.94     1.77861                                         6      -32.79         0.10                                                     7      -277.62        5.56     1.77861                                         8      -34.20         117.36                                                   9      ∞        14.00    1.51462                                         10     -94.57                                                                  ______________________________________                                    

The focal length f, the F-number F_(NO) etc. of the lens system having the aforementioned lens data are set as shown in Table 6.

                  TABLE 6                                                          ______________________________________                                         f        F.sub.NO      λ d.sub.0                                        ______________________________________                                         100.00   20.00         632.8 nm 10.10                                          ______________________________________                                    

The focal length f₂, f₅ of the second and fifth lens L₂, L₅ is -32.8, 183.8, respectively, although this is not shown in Tables 5 and 6.

Thus,

    r.sub.2 /r.sub.3 =2.100

    f.sub.2 /f=-0.328

    f.sub.5 /f=1.838

and it is apparent that the lens system satisfies the inequalities (1) to (3).

FIGS. 9A, 9B and 9C illustrate spherical aberration, astigmatism and f-sinθ characteristic of the lens system having the aforementioned structure, respectively.

Example 4

Table 7 shows lens data of a fourth example according to the lens system.

                  TABLE 7                                                          ______________________________________                                         i      r.sub.1        d.sub.i  n.sub.i                                         ______________________________________                                         1       -36.40        1.55     1.77861                                         2       -17.70        1.82                                                     3       -14.32        5.00     1.61655                                         4      -160.77        2.16                                                     5      -586.93        5.11     1.77861                                         6      -382.79        2.87                                                     7      -385.16        9.44     1.77861                                         8       -31.13        121.82                                                   9      ∞        13.40    1.51462                                         10     -98.98                                                                  ______________________________________                                    

The focal length f, the F-number F_(NO) etc. of the lens system having the aforementioned lens data are set as shown in Table 8.

                  TABLE 8                                                          ______________________________________                                         f        F.sub.NO      λ d.sub.0                                        ______________________________________                                         100.00   20.00         632.8 nm 10.00                                          ______________________________________                                    

The focal length f₂, f₅ of the second and fifth lens L₂, L₅ is -21.1, 192.3, respectively, although this is not shown in Tables 7 and 8.

Thus,

    r.sub.2 /r.sub.3 =1.240

    f.sub.2 /f=-0.211

    f.sub.5 /f=1.923

and it is apparent that the lens system satisfies the inequalities (1) to (3).

FIGS. 10A, 10B and 10C illustrate spherical aberration, astigmatism and f-sinθ characteristic of the lens system having the aforementioned structure, respectively.

Example 5

Table 9 shows lens data of a fifth example according to the lens system.

                  TABLE 9                                                          ______________________________________                                         i       r.sub.1        d.sub.i n.sub.i                                         ______________________________________                                         1       -20.74         3.38    1.77861                                         2       -20.90         10.83                                                   3       -26.41         5.00    1.61655                                         4       5376.85        0.89                                                    5       -120.05        6.91    1.77861                                         6       -49.76         0.10                                                    7       -711.00        3.45    1.77861                                         8       -62.41         67.82                                                   9       ∞        9.46    1.51462                                         10      -87.80                                                                 ______________________________________                                    

The focal length f, the F-number F_(NO) etc. of the lens system having the aforementioned lens data are set as shown in Table 10.

                  TABLE 10                                                         ______________________________________                                         f        F.sub.NO      λ d.sub.0                                        ______________________________________                                         100.00   20.00         632.8 nm 10.00                                          ______________________________________                                    

The focal length f₂, f₅ of the second and fifth lens L₂, L₅ is -42.6, 170.6, respectively, although this is not shown in Tables 9 and 10.

Thus,

    r.sub.2 /r.sub.3 =0.791

    f.sub.2 /f=-0.426

    f.sub.5 /f=1.706

and it is apparent that the lens system satisfies the inequalities (1) to (3).

FIGS. 11A, 11B and 11C illustrate spherical aberration, astigmatism and f-sinθ characteristic of the lens system having the aforementioned structure, respectively.

Example 6

Table 11 shows lens data of a sixth example according to the lens system.

                  TABLE 11                                                         ______________________________________                                         i       r.sub.1        d.sub.i n.sub.i                                         ______________________________________                                         1       -19.75         3.40    1.77861                                         2       -20.13         13.96                                                   3       -25.84         5.00    1.61655                                         4        550.00        1.18                                                    5       -132.88        2.84    1.77861                                         6       -50.21         0.10                                                    7       -190.57        3.50    1.77861                                         8       -51.97         46.55                                                   9       ∞        11.26   1.51462                                         10      -68.45                                                                 ______________________________________                                    

The focal length f, the F-number F_(NO) etc. of the lens system having the aforementioned lens data are set as shown in Table 12.

                  TABLE 12                                                         ______________________________________                                         f        F.sub.NO      λ d.sub.0                                        ______________________________________                                         100.00   20.00         632.8 nm 10.00                                          ______________________________________                                    

The focal length f₂, f₅ of the second and fifth lens L₂, L₅ is -39.9, 133.0, respectively, although this is not shown in Tables 11 and 12.

Thus,

    r.sub.2 / r.sub.3 =0.779

    f.sub.2 /f =-0.399

    f.sub.5 /f =1.330

and it is apparent that the lens system satisfies the inequalities (1) to (3).

FIGS. 12A, 12B and 12C illustrate spherical aberration, astigmatism and f-sinθ characteristic of the lens system having the aforementioned structure, respectively.

Example 7

Table 13 shows lens data of a seventh example according to the lens system.

                  TABLE 13                                                         ______________________________________                                         i      r.sub.1        d.sub.i  n.sub.i                                         ______________________________________                                         1      -29.65         3.73     1.77861                                         2      -19.96         1.63                                                     3      -18.02         5.00     1.61655                                         4       210.58        1.40                                                     5      -174.83        6.02     1.77861                                         6      -60.44         8.26                                                     7      9052.20        4.56     1.77861                                         8      -47.50         124.12                                                   9      ∞        10.81    1.51462                                         10     -133.80                                                                 ______________________________________                                    

The focal length f, the F-number F_(NO) etc. of the lens system having the aforementioned lens data are set as shown in Table 14.

                  TABLE 14                                                         ______________________________________                                         f        F.sub.NO      λ d.sub.0                                        ______________________________________                                         100.01   20.00         632.8 nm 10.00                                          ______________________________________                                    

The focal length f₂, f₅ of the second and fifth lens L₂, L₅ is -26.7, 260.0, respectively, although this is not shown in Tables 13 and 14.

Thus,

    r.sub.2 /r.sub.3 =1.108

    f.sub.2 /f=-0.267

    f.sub.5 /f=2.600

and it is apparent that the lens system satisfies the inequalities (1) to (3).

FIGS. 13A. 13B and 13C illustrate spherical aberration, astigmatism and f-sinθ characteristic of the lens system having the aforementioned structure, respectively.

The spherical aberration of the lens system is extremely small as understood from the spherical aberration diagrams (FIGS. 7A, 8A, 9A, 10A, 11A, 12A, and 13A), and the astigmatism thereof is also extremely small as understood from the astigmatism diagrams (FIGS. 7B, 8B, 9B, 10B, 11B, 12B, and 13B). Further, the lens system structured as above has an excellent f-sinθ characteristic, as understood from the f-sinθ characteristic diagrams (FIGS. 7C, 8C, 9C, 10C, 11C, 12C and 13C).

Further, all the F-numbers F_(NO) of the lens systems are not more than 20 in Examples 1 to 7, as hereinabove described.

FIG. 14 is a perspective view showing an optical system of an optical beam scanner, which scans a plurality of laser beams on a recording surface, to which the aforementioned lens system can be applied. Referring to FIG. 14, a laser beam B₁ outgoing from a laser tube 11 is projected onto a non-polarizing beam splitter 12, to be divided into a scanning beam B₂ and a reference beam B₃.

The scanning beam B₂ reflected by the beam splitter 12 enters an optical modulator system 13 for dividing the scanning beam B₂ into a plurality of laser beams and individually modulating the plurality of laser beams in response to image signals being inputted into the optical modulator system 13. The optical modulator system 13 includes a non-polarizing beam splitter 14 and two polarizing beam splitters 15, 16 so as to divide the scanning beam B₂ into four laser beams B₂₁, B₂₂, B₂₃, B₂₄. The scanning beam B₂ is divided into two laser beams by the beam splitter 14, and then the two laser beams are directed to the beam splitter 15, 16, respectively. The laser beam entering the beam splitter 15 is divided into two laser beams B₂₁, B₂₂ while the laser beam entering the beam splitter 16 is dividied into two laser beam B₂₃, B₂₄, so that the four laser beams B₂₁, B₂₂, B₂₃, B₂₄ then enter optical modulation elements (e.g. acoustic optical modulators) 17 to 20 disposed on optical axes of the laser beams B₂₁, B₂₂, B₂₃, B₂₄, respectively. Thus, the respective laser beams B₂₁, B₂₂, B₂₃, B₂₄ are modulated in response to the image signals from a control unit (not shown) for controlling the optical beam scanner. The structure and operation of the optical modulation elements 17 to 20 are well known to these skilled in the art, and hence redundant description is omitted. Further, two polarizing beam splitter 21, 22 and a non-polarizing beam splitter 23 are provided in the optical modulator system 13, so that modulated laser beams B₂₁ ', B₂₂ ', B₂₃ ', B₂₄ ' are bundled, to generate a multibeam B₄ consisting of them which are not parallel each other.

The multibeam B₄ outgoing from the opitcal modulator system 13 are reflected by reflecting mirrors 24, 25, and then projected onto a polygon mirror 28, which is rotated at a constant angular velocity, through a beam expander 26 and a cylindrical lens 27. A multibeam B₅ deflected by the polygon mirror 28 is focused on a photosensitive material (not shown), which is wound on a rotating cylinder 31, by the lens system 29 as shown in FIG. 6 and a cylindrical lens 30, thereby to record a desired image on the photosensitive material.

On the other hand, the reference beam B₃ is projected onto a reflecting mirror 33 disposed in the vicinity of the cylindrical lens 27 through a beam expander 32. A reference beam B₃ ' reflected by the reflecting mirror 33 enters the polygon mirror 28 through the cylindrical lens 27, and is then directed by the lens system 29 and the cylindrical lens 30 toward a reflecting mirror 34. Thus, the multibeam B₅ impinges on the photosensitive material provided on the rotating cylinder 31, while only the corresponding reference beam B₃ ' is reflected by the reflecting mirror 34 onto a linear encoder 35 which is disposed substantially parallel to the scanning line on the photosensitive material.

The lens system 29 is structured as shown in FIG. 6 and satisfies the above inequalities (1) to (3), so as to have a telecentric characteristic. Therefore, the image is recorded on the photosensitive material in high accuracy. Further, the lens system has also an excellent f-sinθ characteristic. Thus, each scanning locus of the respective laser beams forming the multibeam B₅ is made straight. Consequently, the image can be recorded on the photosensitive material not only in high accuracy but also in high quality.

Although the present invention has been described and illustrated in detail, it is clearly understood that the same is by way of illustration and example only and is not to be taken by way of limitation. The spirit and scope of the present invention should be limited only by the terms of the appended claims. 

What is claimed is:
 1. A lens system for an optical beam scanner which scans a plurality of optical beams on a recording surface, said lens system comprising:(a) a first lens group having an entrance pupil, said first lens group including:(a-1) a first lens in the form of a meniscus having a positive power, said first lens having a concave surface which is directed to said entrance pupil; (a-2) a second lens having a negative power; (a-3) a third lens in the form of a meniscus having a positive power; and (a-4) a fourth lens having a positive power; and (b) a second lens group including a fifth lens having a positive power; wherein said first through fifth lenses are successively disposed in order from said entrance pupil side; and wherein said lens system satisfies the following relationships:

    0.6<(r.sub.2 /r.sub.3)≦2.1

    -0.43<(f.sub.2 /f)<-0.21

    1.3<(f.sub.5 /f)≦2.6

where r₂ is a radius of curvature of a surface of said first lens, which is directed to said recording surface; r₃ is a radius of curvature of a surface of said second lens, which is directed to said entrance pupil; f₂ is a focal length of said second lens; f₅ is a focal length of said fifth lens; and f is a focal length of the lens system.
 2. A lens system in accordance with claim 1, wherein said fifth lens is a plano-convex lens having a flat surface which is directed to said fourth lens.
 3. An optical beam scanner for scanning a plurality of optical beams on a recording surface, said optical beam scanner comprising:a light source for generating the plurality of optical beams; a deflector for deflecting the optical beam outgoing from said light source; and a lens system for imaging the optical beams deflected by said deflector on said recording surface, said lens system comprising: (a) a first lens group having an entrance pupil, which includes: (a-1) a first lens in the form of a meniscus having a positive power, said first lens having a concave surface which is directed to said entrance pupil; (a-2) a second lens having a negative power; (a-3) a third lens in the form of a meniscus having a positive power; and (a-4) a fourth lens having a positive power; and (b) a second lens group including a fifth lens having a positive power; wherein said first through fifth lenses are successively disposed in order from said entrance pupil side; and wherein said lens system satisfies the following relationships:
 0. 6<(r₂ /r₃)≦2.1

    -0.43<(f.sub.2 /f)<-0.21

    1.3<(f.sub.5 /f)≦2.6

where r₂ is a radius of curvature of a surface of said first lens, which is directed to said recording surface; r₃ is a radius of curvature of a surface of said second lens, which is directed to said entrance pupil; f₂ is a focal length of said second lens; f₅ is a focal length of said fifth lens; and f is a focal length of the lens system. 